Finger hovering is becoming a highly desirable feature for mobile applications. With finger tracking capability, it may be unnecessary for a user to touch the screen of a mobile device to control a mobile application executing on the mobile device. The hovering feature can be used for remote gesture control, such as photo sliding or map navigation. Unlike camera, electrical field, and self-capacitance or mutual-capacitance based hovering solutions, an ultrasound based hovering system may transmit specially designed broadband modulated ultrasound patterns from ultrasound transmitters.
Ultrasound finger hovering tracking may have an advantage over electrical field based hovering systems and capacitance based hovering systems. For example, an advantage may be that the ultrasound based hovering system uses a minimum amount of hardware. For example, the ultrasound based hovering system may share microphones used by audio and voice applications. Further, the ultrasound based hovering system may operate with adjustable hovering distance and may also work off-screen. Electrical field types of hovering systems may require special electrode pattern design and circuits. Electrical field based hovering systems may only track the movement of the center of gravity of the whole hand and may be unable to track movement corresponding to only the finger. Moreover, the performance of electrical field types of hovering systems may be affected by device grounding conditions. Additionally, a capacitance based hovering system may require a special touchscreen design and the hovering distance may be small (e.g., 2-3 centimeters).
The ultrasound based hovering system may track finger movements by using multiple ultrasound transmitters and microphones. In an example, transmitters (e.g., speakers) transmit ultrasound waves, and microphones receive the reflected signal pattern from the hovering fingertip. The fingertip coordinates may be calculated using triangulation algorithms.
The accuracy of the ultrasound based hovering system, however, may be affected by a number of factors, such as signal-to-noise ratio and transducer bandwidth. An error source may be due to reflections from the non-fingertip parts of the hand. Unlike an ultrasound pen, a hovering finger suffers from unintended reflections from the palm and knuckles. These reflections can cause triangulation errors and discontinuities of the traces. Another error source may be from the fact that the reflection points on the fingertip for each transmitter and receiver pair are not the same. This may result in traces not being smooth, especially in the z-direction.